The laugh box is a fictional organ that appears in the episode "Funny Pants."
It is a larynx-like body part that allows people to be able to laugh.
Role in episode
Fearing the loss of his laugh box, SpongeBob goes 24 hours without laughing as per Squidward's advice, only to find that he is unable to laugh when he wakes up. After failing to laugh even after consulting his friends Patrick and Sandy, SpongeBob spends the night crying over losing his laugh box.
Eventually, Squidward cannot stand it and tells SpongeBob that he lied to him. They both laugh for a while until SpongeBob leaves and Squidward soon is taken to the hospital, overusing his laugh box after not laughing for too long. SpongeBob volunteers to donate part of his laugh box at the hospital and the doctors replace Squidward's with part of SpongeBob's.
At the end of the episode, it is revealed that Squidward is stuck with SpongeBob's laugh.